The Observed Structure and Precipitation Characteristics of Southeast Atlantic Stratocumulus from Airborne Radar during ORACLES 2016–17

Abstract

During the Observations of Aerosols above Clouds and Their Interactions (ORACLES) 2016 and 2017 field experiments, the Third Generation Advanced Precipitation and Cloud Radar (APR-3) flew aboard the NASA P-3 aircraft taking over 10 million profiles of stratocumulus clouds in the southeast Atlantic Ocean. This study documents cloud structure, precipitation frequency and intensity, and atmospheric stability for each flight during both field experiments. A larger cloud fraction was estimated for 2016, likely due to a larger estimated inversion strength (EIS) in the experiment area (between 6 and 10 K) compared to 2017 where EIS was on average 4–6 K lower. We used an optimal estimation retrieval to derive precipitation rates for all measurable clouds during both experiments. Over 30% of clouds observed during the 2016 experiment exhibited precipitation reaching the surface, but retrieved drizzle rates were below 0.01 mm h−1 in all but 40% of these profiles. This is in sharp contrast to the 2017 campaign where over 53% of precipitating profiles had rainfall rates larger than 0.01 mm h−1. The differences in cloud and rain fractions between the two years are most likely due to differences in the sampling environments; however, enough variations in cloud, virga, and rain fraction exist for similar environmental conditions such that additional analysis of cloud and aerosol interactions—specifically their effect on precipitation processes—needs further exploration. The extensive APR-3 sampling of drizzling stratocumulus under similar thermodynamic conditions provides a rich dataset for examining the influence of biomass burning aerosols on cloud fraction, morphology, and precipitation characteristics in this climatically important region.

Impact of the variability in vertical separation between biomass burning aerosols and marine stratocumulus on cloud microphysical properties over the Southeast Atlantic

“…The changes in N c , R e , and drizzle concentration presented here could lead to aerosol-induced precipitation suppression and impact stratocumulus-to-cumulus transitions over the Southeast Atlantic (Yamaguchi et al, 2015;Zhou et al, 2017). Subsequently, changes in precipitation rate could affect the balance between aerosol scavenging and entrainment and modulate the reversible open-closed-cell transitions (Abel et al, 2020;Feingold et al, 2015). These processes would affect the cloud radiative forcing and the direct aerosol radiative forcing, which depends on the albedo of the underlying cloud layer (Cochrane et al, 2019).…”

An overview of the ORACLES (ObseRvations of Aerosols above CLouds and their intEractionS) project: aerosol-cloud-radiation interactions in the Southeast Atlantic basin

“…• there was ample evidence for aerosol-induced modifications of Sc cloud properties (Gupta et al, in prep. ), and some evidence for the suppression of drizzle (Dzambo et al, 2019); 1180…”

An overview of the ORACLES (ObseRvations of Aerosols above CLouds and their intEractionS) project: aerosol–cloud–radiation interactions in the southeast Atlantic basin

“…Comparing the two campaigns flown out of São Tomé (2017 and 2018), precipitation was significantly lighter in 2017 (Fig. 17), which may indicate suppression due to higher droplet concentrations, but additional analysis of cloud thickness and liquid water path differences between the campaigns is underway to quantify these impacts (see also Dzambo et al, 2020).…”